Method of making imager structure

ABSTRACT

In an imager having an array of light-sensitive elements and employing striped common electrodes, exposed edges of preimidized polyimide layers above the light-sensitive imaging elements are sealed with the material of the common electrode (e.g., indium tin oxide). Similarly, exposed preimidized polyimide edges in electrical contacts for the array and bridge members electrically coupling adjacent light-sensitive imaging elements are also sealed with the material of the common electrode.

This application is a division of application Ser. No. 09/522,231 filedMar. 9, 2000, U.S. Pat. No. 6,465,824 which is hereby incorporated byreference in its entirety.

The United States Government may have certain rights in this inventionpursuant to contract number MD972-94-0028 awarded by the AdvancedResearch Project Agency (ARPA).

BACKGROUND OF THE INVENTION

This invention generally relates to light-sensitive imaging arrays. Moreparticularly, the present invention relates to sealing of exposed edgesof organic dielectric layers to prevent undercutting of the organicdielectric layers from adversely affecting imager performance andreliability.

Photosensitive element arrays for converting incident radiant energyinto an electrical signal are commonly used in imaging applications, forexample, in x-ray imagers and facsimile device arrays. Hydrogenatedamorphous silicon (a-Si) and alloys of a-Si are commonly used in thefabrication of photosensitive elements for such arrays due to theadvantageous characteristics of a-Si and the relative ease offabrication. In particular, photosensitive elements, such asphotodiodes, can be formed from such materials in conjunction withnecessary control or switching elements, such as thin film transistors(TFTs), in relatively large arrays.

X-ray imagers, for example, are formed on a substantially flatsubstrate, typically glass. The imager includes an array of pixels withlight-sensitive imaging elements, typically photodiodes, each of whichhas an associated switching element, such as a TFT or one or moreadditional addressing diodes. In conjunction with a scintillator, x-raysare transformed into visible light for imaging with the photosensitiveelements. The photosensitive elements, typically photodiodes, areconnected at one surface to a switching device, typically a thin-filmtransistor, and at the other surface to a common electrode whichcontacts all the photodiodes in parallel. The array is addressed by aplurality of row and column address lines having contact pads locatedalong the sides of the array. In operation, the voltage on the rowlines, and hence the TFTs, are switched on in turn, allowing the chargeon that scanned line's photodiodes to be read out via the column addresslines, which are connected to external amplifiers. The row address linesare commonly referred to as “scan lines” and the column address linesare referred to as “data lines.” The address lines are electricallycontiguous with contact fingers which extend from the active regiontoward the edges of the substrate, where they are in turn electricallyconnected to contact pads. Connection to external scan line drive anddata line read out circuitry is made via the contact pads.

The common electrode, which is disposed over the top of the photodiodearray provides electrical contact to the photodiode array. Thephotodiode array is typically overlaid with a first layer of inorganicand a second layer of organic polymer dielectric, as disclosed in U.S.Pat. No. 5,233,181, issued on Aug. 3, 1993 to Kwansnick (sic) et al.Contact vias are formed over the photodiodes in each dielectric layer toallow electrical contact to the photodiode tops by the common electrode.

Patterning of the common electrode comprises deposition,photolithography and photoresist strip, as is well known in the art. Forlight imagers comprising amorphous silicon, it is observed that thevias, necessary for electrical connection between the contact pads andthe contact fingers, may be damaged if the photoresist is removed by awet strip process, degrading the imager. Therefore, dry strip of thecommon electrode photoresist is generally used, for example, by ashingwith a plasma containing O₂. However, the dry strip also etches theunderlying organic polymer, causing undercut of its edges under those ofthe common electrode. A barrier layer is typically disposed on theimager after common electrode formation, for example, see U.S. Pat. No.5,401,668, issued Mar. 28, 1999 to Kwasnick et al., and this commonelectrode overhang results in poor step coverage of the barrier layer,causing degraded environmental protection and possible photodiodeleakage. Thus, a need exists to address the undercutting problem.

It is desirable that the imager structure be robust both forwithstanding the fabrication process and good performance in operation.As higher performance is required of imagers (e.g., noise, resolution,etc.), the necessity arises of greater patterning of the imagerstructure to provide the desired performance in operation and ability towithstand the rigors of fabrication and usage.

SUMMARY OF THE INVENTION

In one example of the present invention, a structure and a method offorming the structure for an imager is presented. The structurecomprises an organic dielectric layer, and a common electrode,comprising a light-transmissive conductive layer, the common electrodecovering the organic dielectric layer and extending beyond an exposededge of the organic dielectric layer along a “striped” segment of thecommon electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an imager in accordance with the presentinvention.

FIG. 2 is an exploded view of a segment of the imaging array in theimager of FIG. 1.

FIG. 3 is a cross-sectional view taken along lines 3—3 of a portion ofthe array segment of FIG. 2.

FIGS. 4-7 are cross-sectional views of part of the array segment portionof FIG. 3 during fabrication.

FIG. 8 is a cross-sectional view taken along lines 8—8 of FIG. 2 of abridge member.

FIG. 9 is a close-up view of a portion of the imager of FIG. 1.

FIG. 10 is a cross-sectional view taken along lines 10—10 of the imagerportion of FIG. 9.

FIG. 11 is a cross-sectional view of part 111 of portion 110 taken alonglines 11—11 in FIG. 10.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a plan view of an exemplary imager 10 in accordance with thepresent invention. The imager 10 is typically formed on a substantiallyflat substrate 12, typically glass. The imager 10 includes an array 14of light-sensitive imaging elements, preferably photodiodes, arranged ina matrix, each imaging element having associated therewith a switchingelement, preferably a thin-film transistor (TFT). Both devices(photodiode and TFT) preferably comprise amorphous silicon (a-Si). Thislight sensitive region of the array is typically referred to as theactive region of the array. The array 14 is addressed around itsperimeter by a plurality of row and column address lines having contactpads 16 and 18, respectively, which are located along the sides of array14 as indicated by the dot representations of FIG. 1.

In operation, the voltage on the row lines, and hence the TFTs, areswitched on in turn, allowing the charge on that scanned line'sphotodiodes to be read out via the column address lines. The row addresslines are commonly called the scan lines and the column address linesthe data lines. A data line 32 (a few representative examples of whichare illustrated in FIG. 1) typically extends between each ofcorresponding set of contact pads 18 in the array, which data lines areused for readout of charge accumulated on the photodiode array duringimaging operations.

The address lines are disposed in the active region of the pixel array14, with contact finger 20 extending from the active region towards theedge of the substrate. The contact finger 20 electrically connects tocontact pads, such as row contact pads 16 and column contact pads 18,which, in turn, can be electrically connected to external devices. Asmore fully discussed in U.S. Pat. No. 5,389,775 issued Feb. 14, 1995 ofKwasnick et al., the contact pads include contact pads connected tocommon electrodes of the array.

Outside the contact pads, such as contact pad 16, a guard ring 22 istypically disposed around the perimeter of the pixel array. Guard ring22 is typically maintained at ground potential during operation andserves the purpose of protecting the array from electrostatic dischargeduring the formation of the imager, and during connection of the imagerto external circuitry, and acts as a ground potential for the imager 10.The guard ring 22 has one or more guard contact pads 24 spaced apartfrom each other around the inner side of the perimeter of the guard ring22 as shown in FIG. 1.

A common electrode 38 (a small representative portion of which isillustrated in FIG. 1) is disposed over the array to provide a commoncontact to the tops of each of the diodes in the imager array.Electrical capacitance between the data lines 32 and the commonelectrode 38 can contribute to electrical noise in the read outcircuitry. For low signal applications, such as fluoroscopy in medicalprocedures, and for large imagers with data lines longer than about 20cm, the noise is of a magnitude that imager performance is degraded.Thus, the common electrode typically is removed from the regionoverlaying the data lines 32, reducing the capacitance and therebyimproving imager performance, resulting in a so-called “striped” commonelectrode. The common electrode thus has a plurality of striped segments39 extending across the imager substantially parallel to, but notoverlying, the data lines 32. The respective striped segments 39 arecoupled by a plurality of bridge members 40 (disposed so as to be“cross-striped” in FIG. 1) that bridge over the data line betweenstriped segments.

FIG. 2 is a plan view of a portion 26 of the light-sensitive imagingarray 14 of FIG. 1, including glass substrate 12, adjacentlight-sensitive imaging elements 28 and 30 and data lines 32 and 34.Preferably, elements 28 and 30 are photodiodes. Also shown in phantom isa scan line 36. A striped common electrode 38 is coupled to all thelight-sensitive imaging elements in the array, as well as to bridgemembers (discussed below), and acts as a low resistance electricalreturn path to the photodiodes from external circuitry. The stripedcommon electrode is electrically coupled to the respective photodiodeswith a respective via 43 formed through underlying dielectric layers toenable the common electrode material to come into electrical contactwith the semiconductor material of the photodiode body.

As noted above, the striped common electrode 38 generally runs parallelto, but does not cover, the data lines. Preferably, the common electrodeis situated laterally a distance of at least about 3 microns from thedata lines. Bridge members 40 and 41 electrically couple photodiode 30to adjacent photodiodes (not shown) to the left and right of photodiode30. Although bridge members 40 and 41 electrically couple three adjacentphotodiodes, it will be understood that bridge member groups of more orless immediately adjacent electronically coupled photodiodes could becreated. Although there could be bridge members between all adjacentphotodiodes, they are preferably periodically dispersed in the array,for example, every other photodiode group, or most preferably about tento about twenty photodiodes between groups in a given row in order toreduce data line capacitance.

The bridge members promote electrical re-equilibration in the arrayafter an image is taken. Since the array includes many photodiodes, eachphotodiode will have different voltages during imaging, and current maytend to flow in the direction of the scan lines as well as the datalines. The bridge members help current to flow in the direction of thescan lines.

Since the common electrodes are striped, breaks in the common electrodeor electrical isolation of a diode for repair will sever the connection.The bridge members solve this problem by providing an alternateconnection path should a common electrode stripe develop a break for anyreason. Bridge members generally are described in detail in U.S. Pat.No. 5,777,355, entitled “Radiation Imager with Discontinuous Dielectric”issued to Possin et al. on Jul. 7, 1998.

As used herein, “striped segment” of the common electrode refers tothose portions of the common electrode extending both vertically(segment 39) and horizontally (cross bridge structure 40) as illustratedin FIG. 1, and reference to edges of dielectric material along thestriped segments of the common electrode relate to those edges where thecommon electrode material has been terminated (that is, the conductivematerial has been removed) so as to not overlie data lines 32, and doesnot refer to vias 40 that are formed to enable electrical contactbetween the photodiode and the common electrode. Typically in vias, thecommon electrode material is not terminated, but forms a covering overthe walls of the via and the surface of the semiconductor material ofthe photodiode body that was exposed by formation of the via.

FIG. 3 is a cross-sectional view of a section 42 of portion 26 of FIG. 2taken along line 3—3 through photodiode 28. Section 42 includes datalines 32 and 34 on substrate 12, as well as diode bottom contact pad 46.The data lines typically comprise, for example, molybdenum, aluminum ora tiered combination thereof. Covering the data lines and edges of thediode bottom contact pad is a layer 48 of a passivation dielectric, suchas, for example, silicon oxide deposited, for example, byplasma-enhanced vapor deposition. Over the bottom contact pad isphotodiode 50. Photodiode 50 includes, for example, a bottom layer 52 ofN+ silicon doped with, for example, phosphorous. Above bottom layer 52is, for example, a layer 54 of substantially intrinsic silicon overwhich is a layer 56 of P+ doped silicon using, e.g., boron. Covering theedges of photodiode 50 is a layer 58 of a passivation dielectric, suchas, for example, silicon nitride or silicon oxide. Alternatively, thedual-layer dielectric discussed in U.S. Pat. No. 5,233,181, issued onAug. 3, 1999 to Kwansnick (sic) et al., is used. Covering the inneredges of layer 58 and contacting top layer 56 of photodiode 50 is alayer 60 of a second organic dielectric, preferably preimidizedpolyimide (available from, for example, Arch Chemical, Inc.). Finally,covering the polyimide layer is a light-transmissive conductive layer 62contacting photodiode 50 (here, contacting the top of the photodiode),and serving as the striped common electrode 38. Preferably, layer 62(that forms common electrode 38) is a light-transmissive conductiveoxide, and most preferably, indium tin oxide. The preference fortransparency is to allow for the transmission of light into thephotodiode. As used herein, the term “light-transmissive” means allowingat least about 10% of the incident light to pass through.

The use of polyimide for layer 60 serves three purposes. The firstpurpose is to improve step coverage of subsequent fabrication layers.Polyimide is disposed onto the substrate as a viscous liquid and soforms a conformal coating over layer 58. When patterned by conventionalphotolithographic methods, it forms a gradual slope for the commonelectrode material (i.e., the light-transmissive conductive material)because it etches in O₂ plasma at about the same rate as that ofphotoresist, which is naturally sloped by a postbake done as part ofstandard photolithography. For example, a moisture barrier layer formedover light-transmissive conductive layer 62 shows improved step coverageby using polyimide for organic dielectric layer 60. The second purposeis to help prevent electrical shorts, as polyimide does not tend to formto have pin holes, which are more commonly seen in some inorganicdielectric layers (e.g., silicon oxide). The third purpose for use ofthe polyimide layer is the reduction of electrical capacitance betweenthe common electrode and data lines. As noted above, patterning thecommon electrode 38 into striped segments 39, with connecting bridges40, is also used to reduce undesirable parasitic capacitance between thecommon electrode and the data lines.

In each of FIGS. 2 and 3, the switching element in each pixel, forexample, a thin film transistor, is not shown to simplify the figure forease of understanding the invention.

As applied to array 14, and in particular to representative section 42(FIG. 3), in one embodiment the present invention includes covering atleast a portion of the exposed edges the organic dielectric (here,polyimide) layers 60 and 100 (layer 100 described hereafter) along thestriped segments of the common electrode with the light-transmissiveconductive material (here, indium tin oxide), as explained more fullybelow. For example, polyimide edge 64 is covered with portion 66 ofcommon electrode material layer 62. Covering exposed edges (e.g., thenon-horizontal surfaces of the organic dielectric where it becomesdiscontinuous) serves to seal these edges (that is, coat them) so as toprotect the edge, that is reduce the surface area of the organicdielectric that may be subject to undercutting or other attack by anetching agent.

FIG. 4 depicts a relevant portion 67 above data line 32 of section 42 ofFIG. 3 during the later steps of fabrication. It will be understood thatthe fabrication process over data line 34 is similar. At this stage, aspart of the process of forming the striped segments of the commonelectrode, organic dielectric layer 60 is deposited, and patterned toremove it over the data lines, except where the data lines intersectwith the bridge members 40. Organic dielectric layer 60 has a thicknessof about 0.5 microns to about 2.5 microns. One example of an organicdielectric material that could be used for layer 60 is polyimide,preferably preimidized.

FIG. 5 depicts portion 67 after patterning of organic dielectric layer60. Conventional lithographic techniques (e.g. photoresist) commonly areused to do the patterning.

FIG. 6 depicts portion 67 after deposition of light-transmissiveconductive layer 62, and before patterning of same. Light-transmissiveconductive layer 62 has a thickness of about 500 Å to about 2000 Å, andcomprises indium tin oxide or the like.

FIG. 7 depicts portion 67 from FIG. 6 after a layer of photoresist 68 isdeposited, exposed, and developed for patterning of light-transmissiveconductive layer 62. Once the section of layer 62 not covered byphotoresist 68 is removed, the remaining photoresist must also beremoved.

For example, where the light-transmissive conductive layer 62 is indiumtin oxide, patterning would typically be accomplished using HClcontaining etchants (i.e., a “wet” etch). However, it will be understoodthat other wet etches (or dry etches, such as, for example, reactive ionetching) can be used. Each photoresist removal method has its uniquecharacteristics.

The wet stripping is generally more expensive and has been noted to harmthe vias for the contact pads for connection to off-array components,also described below with regards to FIG. 10, while ashing will attackpolyimide layer 60 and has a tendency to undercut it. Such undercuttingcould lead to step coverage degradation for subsequent depositions, suchas, for example, a barrier layer for a an x-ray imager such that thefunctionality of the barrier layer to protect the photodiodes frommoisture would be degraded. The present invention allows the use ofashing without leaving the organic dielectric layer exposed in anundercut condition.

As one skilled in the art will know, ashing is essentially a “gentle”plasma etching process. One form of etches uses reactive ion etching(RIE). Briefly, the workpiece is placed in a chamber at higher pressurethan conventional RIE with an O₂ plasma. An RF potential creates aplasma which causes the photoresist to be etched. The higher pressurechanges the mean free path of the ions produced, reducing ionbombardment, and in this sense is more gentle. Though not accurate,ashing is often thought of more as burning the material away with anenhanced oxidation process using oxygen ions.

FIG. 8 depicts portion 67 after patterning of light-transmissiveconductive layer 62, and removal of photoresist 68. As shown in FIG. 8,edge 70 of organic dielectric layer 60 is sealed with an extension 72 oflight-transmissive conductive layer 62. Extension 72 extends about 1 toabout 5 microns beyond edge 70. The end result is that depicted in FIG.3.

FIG. 9 is a cross-sectional view of a portion 98 of bridge member 40taken along lines 9—9 of FIG. 2. As shown, each bridge member includes alayer 100 of the organic dielectric material of layer 60 of FIG. 3, anda layer 102 of the light-transmissive conductive material of the commonelectrode. The conductive material covers the edges 104 and 106 of thedielectric material, and extends about 1 to about 5 microns beyond edge104, 106 (that is, beyond the termination region 105 of layer 100) toprovide protection for layer 100.

FIG. 10 depicts a portion 110 of imager 10 from FIG. 1. Portion 110shows details of the connection between contact pad 16 and finger 20.Finger 20 actually runs underneath contact pad 16 and electricallycouples thereto at contact via 21. Contact via 21 typically comprise,for example, a light-transmissive conductive layer (e.g., ITO). Contactpad 16 preferably comprises the same material as the common electrode(preferably, indium tin oxide).

FIG. 11 is a cross-sectional view of part 111 of portion 110 taken alonglines 11—11 in FIG. 10. As shown in FIG. 11, finger 20 over substrate 12comprises a conductive layer 112 (e.g., molybdenum, aluminum, or acombination of tiers of each material) similar to the data lines incomposition. Over the conductive layer is a dual passivation dielectriccomprising layers 114 and 116. Like layer 48 in FIG. 3, layer 114comprises, for example, silicon oxide. Similarly, like layer 58 in FIG.3, layer 116 comprises, for example, silicon nitride or alternativelysilicon oxide, or combination thereof. Over the passivation dielectricsis a layer 118 of an organic dielectric (e.g., polyimide). Layer 118 ispreferably formed at the same time in the fabrication process as layer60 (FIG. 3), and may actually be the same physical layer. Thelight-transmissive conductive layer 23, filling contact via 21, permitselectrical contact between conductive layer 112 of finger 20 and contactpad 16. As with FIG. 3, light-transmissive conductive layer 23 includesextensions 122 and 124 of at least about 1 to about 5 microns coveringthe edges 126 and 128, respectively, of organic dielectric layer 118 toprevent damage thereto. Moreover, layers 23 and 62 (FIG. 3) arepreferably formed at the same time in the fabrication process, and mayactually be the same physical layer.

The sealing of exposed polyimide boundaries as described herein alongthe striped segments of the common electrode can be expanded to anysituation in which an organic dielectric layer may be in danger ofundercutting.

In another example of the present invention, a method of fabricating astructure for an imager is presented. An organic dielectric layer isformed, and covered by a light-transmissive conductive layer. Thelight-transmissive conductive layer is extended beyond an exposed edgeof the organic dielectric layer sufficient to protect the exposed edgein subsequent fabrication of the imager.

In still another example of the present invention, a method offabricating an imager is presented. A layer of light-transmissiveconductive material is formed directly over an organic dielectric layerto act as an electrical contact within the imager. The layer oflight-transmissive conductive material is extended at least about onemicron beyond an exposed edge of the organic dielectric layer.

Although the present invention has been described with reference to theparticular embodiments herein, it will be understood that the techniqueof sealing the edges of the polyimide or other dielectric withlight-transmissive conductive oxide or other conductive material isapplicable in other situations. Thus, alternative aspects may beeffected by those skilled in the art.

What is claimed is:
 1. A method of fabricating a structure for an x-rayimager, comprising: forming an imager array having a pattern pixelscomprising light-sensitive imaging elements with a respective switchingdevice, the pixels being coupled together via rows and columns ofconductive lines; forming an organic dielectric layer over portions ofthe light-sensitive elements and said rows and columns of conductivelines, said organic dielectric layer having sidewall edges where saidorganic dielectric layer is patterned to not extend over said array; andforming a light-transmissive conductive layer disposed to couple saidlight-sensitive imaging elements together, said conductive layer beingpatterned into striped segments with connecting bridges so as to contacteach light-sensitive element, and further disposed to cover the sidewalledges of said organic dielectric layer; the light-transmissiveconductive layer being disposed beyond an exposed edge of the organicdielectric layer sufficient to protect the sidewall edges of saidorganic dielectric layer in subsequent fabrication of the imager.
 2. Themethod of claim 1 wherein forming the organic dielectric layer comprisesforming a polyimide layer.
 3. The method of claim 2, wherein forming thepolyimide layer comprises forming a preimidized polyimide layer.
 4. Themethod of claim 1, wherein forming the light-transmissive conductivelayer comprises forming a light-transmissive conductive oxide layer. 5.The method of claim 4, wherein forming the light-transmissive conductiveoxide layer comprises forming a layer of indium tin oxide.
 6. The methodof claim 1 wherein the extending comprises extending thelight-transmissive conductive layer at least about one micron beyond theexposed edge.
 7. The method of claim 6 wherein the extending comprisesextending the light-transmissive conductive layer between about onemicron and about five microns beyond the exposed edge.
 8. A method offabricating an imager, comprising: forming an imager array having apattern pixels comprising light-sensitive imaging elements with arespective switching device, the pixels being coupled together via rowsand columns of conductive lines; forming a layer of light-transmissiveconductive material directly over an organic dielectric layer to act asa common electrode within the imager, said common electrode beingpatterned into striped segments with connecting bridges so as to contacteach light-sensitive element; and disposing the layer oflight-transmissive conductive material at least about one micron beyondan exposed edge of the organic dielectric layer along patterned segmentsof said common electrode fanned by said layer of light-transmissiveconductive material.
 9. The method of claim 8, wherein the extendingcomprises extending the layer of light-transmissive conductive materialbetween about one micron and about five microns beyond the exposed edge.10. The method of claim 8 wherein said connecting bridges are disposedsuch that about 10 to about 20 of the plurality of light-sensitiveimaging elements separate bridge members in the row or column.
 11. Themethod of claim 8 wherein the method further comprises electricallycoupling the array to circuitry outside the array, and wherein theelectrically coupling comprises forming the layer of light-transmissiveconductive material and extending the layer of light-transmissiveconductive material.